Non-volatile memory and method with reduced source line bias errors

ABSTRACT

Source line bias is an error introduced by a non-zero resistance in the ground loop of the read/write circuits. During sensing the control gate voltage of a memory cell is erroneously biased by a voltage drop across the resistance. This error is minimized when the current flowing though the ground loop is reduced. A method for reducing source line bias is accomplished by read/write circuits with features and techniques for multi-pass sensing. When a page of memory cells are being sensed in parallel, each pass helps to identify and shut down the memory cells with conduction current higher than a given demarcation current value. In this way, sensing in subsequent passes will be less affected by source line bias since the total amount of current flow is significantly reduced by eliminating contributions from the higher current cells.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of application Ser. No. 10/254,830,filed on Sep.24, 2002, now U.S. Pat. No. 7,196,931, which applicationsare incorporated herein in their entirety by this reference.

FIELD OF THE INVENTION

This invention relates generally to non-volatile semiconductor memorysuch as electrically erasable programmable read-only memory (EEPROM) andflash EEPROM, and specifically ones having improved sensing circuits.

BACKGROUND OF THE INVENTION

Solid-state memory capable of nonvolatile storage of charge,particularly in the form of EEPROM and flash EEPROM packaged as a smallform factor card, has recently become the storage of choice in a varietyof mobile and handheld devices, notably information appliances andconsumer electronics products. Unlike RAM (random access memory) that isalso solid-state memory, flash memory is non-volatile, retaining itsstored data even after power is turned off. In spite of the higher cost,flash memory is increasingly being used in mass storage applications.Conventional mass storage, based on rotating magnetic medium such ashard drives and floppy disks, is unsuitable for the mobile and handheldenvironment. This is because disk drives tend to be bulky, are prone tomechanical failure and have high latency and high power requirements.These undesirable attributes make disk-based storage impractical in mostmobile and portable applications. On the other hand, flash memory, bothembedded and in the form of a removable card is ideally suited in themobile and handheld environment because of its small size, low powerconsumption, high speed and high reliability features.

EEPROM and electrically programmable read-only memory (EPROM) arenon-volatile memory that can be erased and have new data written or“programmed” into their memory cells. Both utilize a floating(unconnected) conductive gate, in a field effect transistor structure,positioned over a channel region in a semiconductor substrate, betweensource and drain regions. A control gate is then provided over thefloating gate. The threshold voltage characteristic of the transistor iscontrolled by the amount of charge that is retained on the floatinggate. That is, for a given level of charge on the floating gate, thereis a corresponding voltage (threshold) that must be applied to thecontrol gate before the transistor is turned “on” to permit conductionbetween its source and drain regions.

The floating gate can hold a range of charges and therefore can beprogrammed to any threshold voltage level within a threshold voltagewindow. The size of the threshold voltage window is delimited by theminimum and maximum threshold levels of the device, which in turncorrespond to the range of the charges that can be programmed onto thefloating gate. The threshold window generally depends on the memorydevice's characteristics, operating conditions and history. Eachdistinct, resolvable threshold voltage level range within the windowmay, in principle, be used to designate a definite memory state of thecell.

The transistor serving as a memory cell is typically programmed to a“programmed” state by one of two mechanisms. In “hot electroninjection,” a high voltage applied to the drain accelerates electronsacross the substrate channel region. At the same time a high voltageapplied to the control gate pulls the hot electrons through a thin gatedielectric onto the floating gate. In “tunneling injection,” a highvoltage is applied to the control gate relative to the substrate. Inthis way, electrons are pulled from the substrate to the interveningfloating gate.

The memory device may be erased by a number of mechanisms. For EPROM,the memory is bulk erasable by removing the charge from the floatinggate by ultraviolet radiation. For EEPROM, a memory cell is electricallyerasable, by applying a high voltage to the substrate relative to thecontrol gate so as to induce electrons in the floating gate to tunnelthrough a thin oxide to the substrate channel region (i.e.,Fowler-Nordheim tunneling.) Typically, the EEPROM is erasable byte bybyte. For flash EEPROM, the memory is electrically erasable either allat once or one or more blocks at a time, where a block may consist of512 bytes or more of memory.

EXAMPLES OF NON-VOLATILE MEMORY CELLS

The memory devices typically comprise one or more memory chips that maybe mounted on a card. Each memory chip comprises an array of memorycells supported by peripheral circuits such as decoders and erase, writeand read circuits. The more sophisticated memory devices also come witha controller that performs intelligent and higher level memoryoperations and interfacing. There are many commercially successfulnon-volatile solid-state memory devices being used today. These memorydevices may employ different types of memory cells, each type having oneor more charge storage element.

FIGS. 1A-1E illustrate schematically different examples of non-volatilememory cells.

FIG. 1A illustrates schematically a non-volatile memory in the form ofan EEPROM cell with a floating gate for storing charge. An electricallyerasable and programmable read-only memory (EEPROM) has a similarstructure to EPROM, but additionally provides a mechanism for loadingand removing charge electrically from its floating gate upon applicationof proper voltages without the need for exposure to UV radiation.Examples of such cells and methods of manufacturing them are given inU.S. Pat. No. 5,595,924.

FIG. 1B illustrates schematically a flash EEPROM cell having both aselect gate and a control or steering gate. The memory cell 10 has a“split-channel” 12 between source 14 and drain 16 diffusions. A cell isformed effectively with two transistors T1 and T2 in series. T1 servesas a memory transistor having a floating gate 20 and a control gate 30.The floating gate is capable of storing a selectable amount of charge.The amount of current that can flow through the T1's portion of thechannel depends on the voltage on the control gate 30 and the amount ofcharge residing on the intervening floating gate 20. T2 serves as aselect transistor having a select gate 40. When T2 is turned on by avoltage at the select gate 40, it allows the current in the T1's portionof the channel to pass between the source and drain. The selecttransistor provides a switch along the source-drain channel independentof the voltage at the control gate. One advantage is that it can be usedto turn off those cells that are still conducting at zero control gatevoltage due to their charge depletion (positive) at their floatinggates. The other advantage is that it allows source side injectionprogramming to be more easily implemented.

One simple embodiment of the split-channel memory cell is where theselect gate and the control gate are connected to the same word line asindicated schematically by a dotted line shown in FIG. 1B. This isaccomplished by having a charge storage element (floating gate)positioned over one portion of the channel and a control gate structure(which is part of a word line) positioned over the other channel portionas well as over the charge storage element. This effectively forms acell with two transistors in series, one (the memory transistor) with acombination of the amount of charge on the charge storage element andthe voltage on the word line controlling the amount of current that canflow through its portion of the channel, and the other (the selecttransistor) having the word line alone serving as its gate. Examples ofsuch cells, their uses in memory systems and methods of manufacturingthem are given in U.S. Pat. Nos. 5,070,032, 5,095,344, 5,315,541,5,343,063, and 5,661,053.

A more refined embodiment of the split-channel cell shown in FIG. 1B iswhen the select gate and the control gate are independent and notconnected by the dotted line between them. One implementation has thecontrol gates of one column in an array of cells connected to a control(or steering) line perpendicular to the word line. The effect is torelieve the word line from having to perform two functions at the sametime when reading or programming a selected cell. Those two functionsare (1) to serve as a gate of a select transistor, thus requiring aproper voltage to turn the select transistor on and off, and (2) todrive the voltage of the charge storage element to a desired levelthrough an electric field (capacitive) coupling between the word lineand the charge storage element. It is often difficult to perform both ofthese functions in an optimum manner with a single voltage. With theseparate control of the control gate and the select gate, the word lineneed only perform function (1), while the added control line performsfunction (2). This capability allows for design of higher performanceprogramming where the programming voltage is geared to the targeteddata. The use of independent control (or steering) gates in a flashEEPROM array is described, for example, in U.S. Pat. Nos. 5,313,421 and6,222,762.

FIG. 1C illustrates schematically another flash EEPROM cell having dualfloating gates and independent select and control gates. The memory cell10 is similar to that of FIG. 1B except it effectively has threetransistors in series. In this type of cell, two storage elements (i.e.,that of T1-left and T1-right) are included over its channel betweensource and drain diffusions with a select transistor T1 in between them.The memory transistors have floating gates 20 and 20′, and control gates30 and 30′, respectively. The select transistor T2 is controlled by aselect gate 40. At any one time, only one of the pair of memorytransistors is accessed for read or write. When the storage unit T1-leftis being accessed, both the T2 and T1-right are turned on to allow thecurrent in the T1-left's portion of the channel to pass between thesource and the drain. Similarly, when the storage unit T1-right is beingaccessed, T2 and T1-left are turned on. Erase is effected by having aportion of the select gate polysilicon in close proximity to thefloating gate and applying a substantial positive voltage (e.g. 20 V) tothe select gate so that the electrons stored within the floating gatecan tunnel to the select gate polysilicon.

FIG. 1D illustrates schematically a string of memory cells organizedinto a NAND cell. An NAND cell 50 consists of a series of memorytransistors M1, M2, . . . Mn (n−4, 8, 16 or higher) daisy-chained bytheir sources and drains. A pair of select transistors S1, S2 controlsthe memory transistors chain's connection to the external via the NANDcell's source terminal 54 and drain terminal 56. In a memory array, whenthe source select transistor S1 is turned on, the source terminal iscoupled to a source line. Similarly, when the drain select transistor S2is turned on, the drain terminal of the NAND cell is coupled to a bitline of the memory array. Each memory transistor in the chain has acharge storage element to store a given amount of charge so as torepresent an intended memory state. A control gate of each memorytransistor provides control over read and write operations. A controlgate of each of the select transistors S1, S2 provides control access tothe NAND cell via its source terminal 54 and drain terminal 56respectively.

When an addressed memory transistor within a NAND cell is read andverified during programming, its control gate is supplied with anappropriate voltage. At the same time, the rest of the non-addressedmemory transistors in the NAND cell 50 are fully turned on byapplication of sufficient voltage on their control gates. In this way, aconductive path is effective created from the source of the individualmemory transistor to the source terminal 54 of the NAND cell andlikewise for the drain of the individual memory transistor to the drainterminal 56 of the cell. Memory devices with such NAND cell structuresare described in U.S. Pat. Nos. 5,570,315, 5,903,495, 6,046,935.”

FIG. 1E illustrates schematically a non-volatile memory with adielectric layer for storing charge. Instead of the conductive floatinggate elements described earlier, a dielectric layer is used. Such memorydevices utilizing dielectric storage element have been described byEitan et al., “NROM: A Novel Localized Trapping, 2-Bit NonvolatileMemory Cell,” IEEE Electron Device Letters, vol. 21, no. 11, November2000, pp. 543-545. An ONO dielectric layer extends across the channelbetween source and drain diffusions. The charge for one data bit islocalized in the dielectric layer adjacent to the drain, and the chargefor the other data bit is localized in the dielectric layer adjacent tothe source. For example, U.S. Pat. Nos. 5,768,192 and 6,011,725 disclosea nonvolatile memory cell having a trapping dielectric sandwichedbetween two silicon dioxide layers. Multi-state data storage isimplemented by separately reading the binary states of the spatiallyseparated charge storage regions within the dielectric.

Memory Array

A memory device typically comprises of a two-dimensional array of memorycells arranged in rows and columns and addressable by word lines and bitlines. The array can be formed according to an NOR type or an NAND typearchitecture.

NOR Array

FIG. 2 illustrates an example of an NOR array of memory cells. Memorydevices with an NOR type architecture have been implemented with cellsof the type illustrated in FIG. 1B or 1C. Each row of memory cells areconnected by their sources and drains in a daisy-chain manner. Thisdesign is sometimes referred to as a virtual ground design. Each memorycell 10 has a source 14, a drain 16, a control gate 30 and a select gate40. The cells in a row have their select gates connected to word line42. The cells in a column have their sources and drains respectivelyconnected to selected bit lines 34 and 36. In some embodiments where thememory cells have their control gate and select gate controlledindependently, a steering line 36 also connects the control gates of thecells in a column.

Many flash EEPROM devices are implemented with memory cells where eachis formed with its control gate and select gate connected together. Inthis case, there is no need for steering lines and a word line simplyconnects all the control gates and select gates of cells along each row.Examples of these designs are disclosed in U.S. Pat. Nos. 5,172,338 and5,418,752. In these designs, the word line essentially performed twofunctions: row selection and supplying control gate voltage to all cellsin the row for reading or programming.

NAND Array

FIG. 3 illustrates an example of an NAND array of memory cells, such asthat shown in FIG. 1D. Along each column of NAND cells, a bit line iscoupled to the drain terminal 56 of each NAND cell. Along each row ofNAND cells, a source line may connect all their source terminals 54.Also the control gates of the NAND cells along a row are connected to aseries of corresponding word lines. An entire row of NAND cells can beaddressed by turning on the pair of select transistors (see FIG. 1D)with appropriate voltages on their control gates via the connected wordlines. When a memory transistor within the chain of a NAND cell is beingread, the remaining memory transistors in the chain are turned on hardvia their associated word lines so that the current flowing through thechain is essentially dependent upon the level of charge stored in thecell being read. An example of an NAND architecture array and itsoperation as part of a memory system is found in U.S. Pat. Nos.5,570,315, 5,774,397 and 6,046,935.

Block Erase

Programming of charge storage memory devices can only result in addingmore charge to its charge storage elements. Therefore, prior to aprogram operation, existing charge in a charge storage element must beremoved (or erased). Erase circuits (not shown) are provided to eraseone or more blocks of memory cells. A non-volatile memory such as EEPROMis referred to as a “Flash” EEPROM when an entire array of cells, orsignificant groups of cells of the array, is electrically erasedtogether (i.e., in a flash). Once erased, the group of cells can then bereprogrammed. The group of cells erasable together may consist one ormore addressable erase unit. The erase unit or block typically storesone or more pages of data, the page being the unit of programming andreading, although more than one page may be programmed or read in asingle operation. Each page typically stores one or more sectors ofdata, the size of the sector being defined by the host system. Anexample is a sector of 512 bytes of user data, following a standardestablished with magnetic disk drives, plus some number of bytes ofoverhead information about the user data and/or the block in with it isstored.

Read/Write Circuits

In the usual two-state EEPROM cell, at least one current breakpointlevel is established so as to partition the conduction window into tworegions. When a cell is read by applying predetermined, fixed voltages,its source/drain current is resolved into a memory state by comparingwith the breakpoint level (or reference current I_(REF)). If the currentread is higher than that of the breakpoint level or I_(REF), the cell isdetermined to be in one logical state (e.g., a “zero” state. On theother hand, if the current is less than that of the breakpoint level,the cell is determined to be in the other logical state (e.g., a “one”state). Thus, such a two-state cell stores one bit of digitalinformation. A reference current source, which may be externallyprogrammable, is often provided as part of a memory system to generatethe breakpoint level current.

In order to increase memory capacity, flash EEPROM devices are beingfabricated with higher and higher density as the state of thesemiconductor technology advances. Another method for increasing storagecapacity is to have each memory cell store more than two states.

For a multi-state or multi-level EEPROM memory cell, the conductionwindow is partitioned into more than two regions by more than onebreakpoint such that each cell is capable of storing more than one bitof data. The information that a given EEPROM array can store is thusincreased with the number of states that each cell can store. EEPROM orflash EEPROM with multi-state or multi-level memory cells have beendescribed in U.S. Pat. No. 5,172,338.

In practice, the memory state of a cell is usually read by sensing theconduction current across the source and drain electrodes of the cellwhen a reference voltage is applied to the control gate. Thus, for eachgiven charge on the floating gate of a cell, a corresponding conductioncurrent with respect to a fixed reference control gate voltage may bedetected. Similarly, the range of charge programmable onto the floatinggate defines a corresponding threshold voltage window or a correspondingconduction current window.

Alternatively, instead of detecting the conduction current among apartitioned current window, it is possible to set the threshold voltagefor a given memory state under test at the control gate and detect ifthe conduction current is lower or higher than a threshold current. Inone implementation the detection of the conduction current relative to athreshold current is accomplished by examining the rate the conductioncurrent is discharging through the capacitance of the bit line.

FIG. 4 illustrates the relation between the source-drain current I_(D)and the control gate voltage V_(CG) for four different charges Q1-Q4that the floating gate may be selectively storing at any one time. Thefour solid I_(D) versus V_(CG) curves represent four possible chargelevels that can be programmed on a floating gate of a memory cell,respectively corresponding to four possible memory states. As anexample, the threshold voltage window of a population of cells may rangefrom 0.5V to 3.5V. Six memory states may be demarcated by partitioningthe threshold window into five regions in interval of 0.5V each. Forexample, if a reference current, I_(REF) of 2 μA is used as shown, thenthe cell programmed with Q1 may be considered to be in a memory state“1” since its curve intersects with IREF in the region of the thresholdwindow demarcated by V_(CG)=0.5V and 1.0V. Similarly, Q4 is in a memorystate “5”.

As can be seen from the description above, the more states a memory cellis made to store, the more finely divided is its threshold window. Thiswill require higher precision in programming and reading operations inorder to be able to achieve the required resolution.

U.S. Pat. No. 4,357,685 discloses a method of programming a 2-stateEPROM in which when a cell is programmed to a given state, it is subjectto successive programming voltage pulses, each time adding incrementalcharge to the floating gate. In between pulses, the cell is read back orverified to determine its source-drain current relative to thebreakpoint level. Programming stops when the current state has beenverified to reach the desired state. The programming pulse train usedmay have increasing period or amplitude.

Prior art programming circuits simply apply programming pulses to stepthrough the threshold window from the erased or ground state until thetarget state is reached. Practically, to allow for adequate resolution,each partitioned or demarcated region would require at least about fiveprogramming steps to transverse. The performance is acceptable for2-state memory cells. However, for multi-state cells, the number ofsteps required increases with the number of partitions and therefore,the programming precision or resolution must be increased. For example,a 16-state cell may require on average at least 40 programming pulses toprogram to a target state.

FIG. 5 illustrates schematically a memory device with a typicalarrangement of a memory array 100 accessible by read/write circuits 170via row decoder 130 and column decoder 160. As described in connectionwith FIGS. 2 and 3, a memory transistor of a memory cell in the memoryarray 100 is addressable via a set of selected word line(s) and bitline(s). The row decoder 130 selects one or more word lines and thecolumn decoder 160 selects one or more bit lines in order to applyappropriate voltages to the respective gates of the addressed memorytransistor. Read/write circuits 170 are provided to read or write(program) the memory states of addressed memory transistors. Theread/write circuits 170 comprise a number of read/write modulesconnectable via bit lines to the memory elements in the array.

Factors Affecting Read/Write Performance and Accuracy

In order to improve read and program performance, multiple chargestorage elements or memory transistors in an array are read orprogrammed in parallel. Thus, a logical “page” of memory elements areread or programmed together. In existing memory architectures, a rowtypically contains several interleaved pages. All memory elements of apage will be read or programmed together. The column decoder willselectively connect each one of the interleaved pages to a correspondingnumber of read/write modules. For example, in one implementation, thememory array is designed to have a page size of 532 bytes (512 bytesplus 20 bytes of overheads.) If each column contains a drain bit lineand there are two interleaved pages per row, this amounts to 8512columns with each page being associated with 4256 columns. There will be4256 sense modules connectable to read or write in parallel either allthe even bit lines or the odd bit lines. In this way, a page of 4256bits (i.e., 532 bytes) of data in parallel are read from or programmedinto the page of memory elements. The read/write modules forming theread/write circuits 170 can be arranged into various architectures.

As mentioned before, conventional memory devices improve read/writeoperations by operating in a massively parallel manner. This approachimproves performances but also has repercussions on the accuracy of readand write operations.

One issue is the source line bias error. This is particular acute formemory architecture where a large number memory cells have their sourcestie together in a source line to ground. Parallel sensing of thesememory cells with common source results in a substantial current throughthe source line. Owing to finite resistance in the source line, this inturn results in an appreciable potential difference between the trueground and the source electrode of each memory cell. During sensing, thethreshold voltage supplied to the control gate of each memory cell isrelative to its source electrode but the system power supply is relativeto the true ground. Thus sensing may become inaccurate due to theexistence of the source line bias error.

Another issue has to do with bit line to bit line coupling or crosstalk.This problem becomes more acute with parallel sensing of closely spacedbit lines. A conventional solution to avoid bit line to bit linecrosstalk is to sense either all even or all odd bit lines at a timewhile grounding the other bit lines. This architecture of a rowconsisting of two interleaved pages will help to avoid bit linecrosstalk as well as to alleviate the problem of densely fitting thepage of read/write circuits. A page decoder is used to multiplex the setof read/write modules to either the even page or the odd page. In thisway, whenever one set of bit lines are being read or programmed, theinterleaving set can be grounded to eliminate crosstalk between odd andeven bit lines, but not between odd lines or even lines.

However, the interleaving page architecture is disadvantageous in atleast three respects. First, it requires additional multiplexingcircuitry. Secondly, it is slow in performance. To finish read orprogram of memory cells connected by a word line or in a row, two reador two program operations are required. Thirdly, it is also not optimumin addressing other disturb effects such as field coupling betweenneighboring charge storage elements at the floating gate level when thetwo neighbors are programmed at different times, such as separately inodd and even pages.

The problem of neighboring field coupling becomes more pronounced withever closer spacing between memory transistors. In a memory transistor,a charge storage element is sandwiched between a channel region and acontrol gate. The current that flows in the channel region is a functionof the resultant electric field contributed by the field at the controlgate and the charge storage element. With ever increasing density,memory transistors are formed closer and closer together. The field fromneighboring charge elements then becomes significant contributor to theresultant field of an affected cell. The neighboring field depends onthe charge programmed into the charge storage elements of the neighbors.This perturbing field is dynamic in nature as it changes with theprogrammed states of the neighbors. Thus, an affected cell may readdifferently at different time depending on the changing states of theneighbors.

The conventional architecture of interleaving page exacerbates the errorcaused by neighboring floating gate coupling. Since the even page andthe odd page are programmed and read independently of each other, a pagemay be programmed under one set of condition but read back under anentirely different set of condition, depending on what has happened tothe intervening page in the meantime. The read errors will become moresevere with increasing density, requiring a more accurate read operationand coarser partitioning of the threshold window for multi-stateimplementation. Performance will suffer and the potential capacity in amulti-state implementation is limited.

Therefore there is a general need for high performance and high capacitynon-volatile memory. In particular, there is a need to have a highcapacity non-volatile memory with improved read and program performancethat effectively manages the aforementioned problems.

SUMMARY OF INVENTION

These needs for a high capacity and high performance non-volatile memorydevice are met by having a large page of read/write circuits to read andwrite a corresponding page of memory cells in parallel. In particular,disturbance effects inherent in high density chip integration are thatmay introduce errors into reading and programming are either eliminatedor minimized.

Source line bias is an error introduced by a non-zero resistance in theground loop of the read/write circuits. The error is caused by a voltagedrop across the resistance when current flows. According to one aspectof the invention, a method for reducing source line bias is accomplishedby read/write circuits with features and techniques for multi-passsensing. When a page of memory cells are being sensed in parallel, eachpass helps to identify and shut down the memory cells with conductioncurrent higher than a given demarcation current value. The identifiedmemory cells are shut down by pulling their associated bit lines toground.

In one implementation, the given demarcation current value is higherthan the breakpoint current value for a conventional single-passsensing. Alternatively, the given demarcation current valueprogressively converges to the breakpoint current value for a conventionsingle pass sensing. In this way, sensing in subsequent passes will beless affected by source line bias since the total amount of current flowis significantly reduced by eliminating the contributions from thehigher current cells.

According to one preferred embodiment, the current states are identifiedin a first pass by comparing each of their conduction currents with thegiven demarcation current value.

According to another preferred embodiment, the higher current states areidentified in first pass by precharging each bit line with a controlledcurrent source. This is accomplished by a precharge circuit acting as acontrolled current source with the supplied current limited to thedemarcation current value. In this way, memory cells with conductioncurrents that exceed the demarcation current value will drain away thecurrent faster than the precharge circuit can charge up their associatedbit lines. Thus, these high current memory cells are identified becausetheir bit lines failed to be charged up and are then eliminated fromparticipating in subsequent passes.

According to yet another preferred embodiment, the higher current statesare identified by multiple passes that include comparison with a givendemarcation current value and controlled precharging.

Another error is caused by capacitive coupling between bit lines.According to another aspect of the present invention, a memory deviceand a method thereof allow sensing a plurality of memory cells inparallel while minimizing errors caused by bit-line to bit-line couplingor crosstalk. Essentially, the plurality of bit line being sensed inparallel has their bit line voltages controlled such that the voltagedifference between each adjacent pair of bit lines is substantiallyindependent of time while their conduction currents are being sensed.When this condition is imposed, all displacement currents due to thevarious bit lines' capacitance drop out since they all depend on a timevarying voltage difference.

In a preferred embodiment this is accomplished by parallel sensingcircuits that also ensures that the potential difference on any adjacentpair of connected bit lines is time-independent.

Prior art sensing includes determining the rate of the conductioncurrent discharging the equivalent capacitor due to the bit linecapacitance. This will contradict the present inventive feature ofsensing at clamped bit line voltage.

According to another aspect of the present invention, a sensing circuitand method allow determination of a memory cell's conduction current bynoting the rate it discharges or charges a given capacitor independentof the bit line. This will allow an optimum sensing circuit and methodto be used, which are independent of the architecture of the memoryarray (i.e., independent of the bit line capacitance.) More importantly,it allows the bit line voltages to be clamped during sensing in order toavoid bit line crosstalk.

An error inherent in a non-volatile memory formed as a high densityintegrated circuit is due to coupling of the field from neighboringcharge storage elements. Individual memory cells are not only affectedby the field from their own storage element but also from that ofneighboring cells. According to another aspect of the present invention,the errors due to the extraneous neighboring fields are minimized byminimizing the change in the field environment of each cell betweenprogramming and reading. This is accomplished by programming alladjacent memory cells in a page thereof together. Since the individualmemory cells and their neighbors are programmed together, it will ensurea minimum change in field environment seen by the individual cells fromthe time they are programmed to the time they are read. In this way theerror incurred during program verified is offset by a similar errorduring read, and the error is reduced and less data-dependent.

Additional features and advantages of the present invention will beunderstood from the following description of its preferred embodiments,which description should be taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1E illustrate schematically different examples of non-volatilememory cells.

FIG. 2 illustrates an example of an NOR array of memory cells.

FIG. 3 illustrates an example of an NAND array of memory cells, such asthat shown in FIG. 1D.

FIG. 4 illustrates the relation between the source-drain current and thecontrol gate voltage for four different charges Q1-Q4 that the floatinggate may be storing at any one time.

FIG. 5 illustrates schematically a typical arrangement of a memory arrayaccessible by read/write circuits via row and column decoders.

FIG. 6A illustrates schematically a memory device having read/writecircuits for reading and programming a page of memory cells in parallel,according to one embodiment of the present invention.

FIG. 6B illustrates a preferred arrangement of the memory device shownin FIG. 6A.

FIG. 7A illustrates the problem of source voltage error due to currentflow in the source line having a finite resistance to ground.

FIG. 7B illustrates the error in the threshold voltage level of a memorycell caused by a source line voltage drop.

FIG. 8 illustrates an example population distribution of a page ofmemory cells for a 4-state memory.

FIG. 9 is a flow diagram showing a multi-pass sensing method forreducing source line bias, according to one embodiment of the invention.

FIG. 10 is a schematic diagram illustrating a multi-pass sense module,according to a preferred embodiment of the invention.

FIG. 11 is a flow diagram showing the operation of the multi-pass sensemodule of FIG. 10.

FIG. 12 illustrates three adjacent bit lines and the effect ofcapacitive couplings between them.

FIG. 13A is a flow diagram showing a method of sensing while reducingbit-line to bit-line coupling.

FIG. 13B is a flow diagram showing a more detailed embodiment of thesensing step shown in FIG. 13A.

FIG. 14 illustrates a preferred sense module implementing the variousaspects of the present invention.

FIG. 15 (A)-FIG. 15(K) are timing diagrams for the sense module shown inFIG. 14.

FIG. 16A is a flow diagram showing a method of programming and readingthat reduces the errors due to neighboring floating gate coupling.

FIG. 16B is a flow diagram showing a preferred embodiment of theinventive step shown in FIG. 16A.

FIG. 17 illustrates a memory array similar to that shown in FIGS. 6A and6B, except its architecture has each row of memory cells organized intoa left page and a right page of memory cells.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 6A illustrates schematically a memory device having read/writecircuits for reading and programming a page of memory cells in parallel,according to one embodiment of the present invention. The memory deviceincludes a two-dimensional array of memory cells 300, control circuitry310, and read/write circuits 370. The memory array 300 is addressable byword lines via a row decoder 330 and by bit lines via a column decoder360. The read/write circuits 370 include multiple sense modules 380 andallows a page of memory cells to be read or programmed in parallel. Inone embodiment, where a row of memory cells are partitioned intomultiple pages, a page multiplexer 350 is provided to multiplex theread/write circuits 370 to the individual pages.

The control circuitry 310 cooperates with the read/write circuits 370 toperform memory operations on the memory array 300. The control circuitry310 includes a state machine 312, an on-chip address decoder 314 and apower control module 316. The state machine 312 provides chip levelcontrol of memory operations. The on-chip address decoder 314 providesan address interface between that used by the host or a memorycontroller to the hardware address used by the decoders 330 and 370. Thepower control module 316 controls the power and voltages supplied to theword lines and bit lines during memory operations.

FIG. 6B illustrates a preferred arrangement of the compact memory deviceshown in FIG. 6A. Access to the memory array 300 by the variousperipheral circuits is implemented in a symmetric fashion, on oppositesides of the array so that the densities of access lines and circuitryon each side are reduced in half. Thus, the row decoder is split intorow decoders 330A and 330B and the column decoder into column decoders360A and 360B. In the embodiment where a row of memory cells arepartitioned into multiple pages, the page multiplexer 350 is split intopage multiplexers 350A and 350B. Similarly, the read/write circuits aresplit into read/write circuits 370A connecting to bit lines from thebottom and read/write circuits 370B connecting to bit lines from the topof the array 300. In this way, the density of the read/write modules,and therefore that of the sense modules 380, is essentially reduced byone half.

Source Line Error Management

One potential problem with sensing memory cells is source line bias.When a large number memory cells are sensed in parallel, their combinecurrents can result in significant voltage drop in a ground loop withfinite resistance. This results in a source line bias which causes errorin a read operation employing threshold voltage sensing.

FIG. 7A illustrates the problem of source voltage error due to currentflow in the source line having a finite resistance to ground. Theread/write circuits 370 operate on a page of memory cellssimultaneously. Each sense modules 380 in the read/write circuits iscoupled to a corresponding cell via a bit line 36. For example, a sensemodule 380 senses the conduction current i₁ (source-drain current) of amemory cell 10. The conduction current flows from the sense modulethrough the bit line 36 into the drain of the memory cell 10 and outfrom the source 14 before going through a source line 34 to ground. Inan integrated circuit chip, the sources of the cells in a memory arrayare all tied together as multiple branches of the source line 34connected to some external ground pad (e.g. Vss pad) of the memory chip.Even when metal strappings are used to reduce the resistance of thesource line, a finite resistance, R, remains between the sourceelectrode of a memory cell and the ground pad. Typically, the groundloop resistance R is around 50 ohm.

For the entire page of memory being sensed in parallel, the totalcurrent flowing through the source line 34 is the sum of all theconduction currents, i.e. i_(TOT)=i₁+i₂+ . . . , +i_(p). Generally eachmemory cell has a conduction current dependent on the amount of chargeprogrammed into its charge storage element. For a given control gatevoltage of the memory cell, a small charge will yield a comparativelyhigher conduction current (see FIG. 4.) When a finite resistance existsbetween the source electrode of a memory cell and the ground pad, thevoltage drop across the resistance is given by V_(drop)=i_(TOT)R.

For example, if 4,256 bit lines discharge at the same time, each with acurrent of 1 μA, then the source line voltage drop will be equal to 4000lines×1 μA/line×50 ohms˜0.2 volts. This source line bias will contributeto a sensing error of 0.2 volts when threshold voltages of the memorycells are sensed.

FIG. 7B illustrates the error in the threshold voltage level of a memorycell caused by a source line voltage drop. The threshold voltage V_(T)supplied to the control gate 30 of the memory cell 10 is relative toGND. However, the effective V_(T) seen by the memory cell is the voltagedifference between its control gate 30 and source 14. There is adifference of approximately V_(drop) between the supplied and effectiveV_(T) (ignoring the smaller contribution of voltage drop from the source14 to the source line.) This V_(drop) or source line bias willcontribute to a sensing error of, for example 0.2 volts when thresholdvoltages of the memory cells are sensed. This bias cannot be easilyremoved as it is data-dependent, i.e., dependent on the memory states ofthe memory cells of the page.

According to one aspect of the invention, a method for reducing sourceline bias is accomplished by read/write circuits with features andtechniques for multi-pass sensing. Each pass helps to identify and shutdown the memory cells with conduction current higher than a givendemarcation current value. Typically with each pass, the givendemarcation current value progressively converges to the breakpointcurrent value for a convention single pass sensing. In this way, sensingin subsequent passes will be less affected by source line bias since thehigher current cells have been shut down.

FIG. 8 illustrates an example population distribution of a page ofmemory cells for a 4-state memory. Each cluster of memory state isprogrammed within a range of conduction currents I_(SD) clearlyseparated from each other. For example, a breakpoint 381 is ademarcating current value between two clusters, respectivelyrepresenting the “1” and “2” memory states. In a conventionalsingle-pass sensing, a necessary condition for a “2” memory state willbe that it has a conduction current less than the breakpoint 381. InFIG. 8, if there were no source line bias, the population distributionwith respect to the supplied threshold voltage V_(T) will be given bythe curve with the solid line. However, because of the source line biaserror, the threshold voltage of each of the memory cells at its controlgate is increased by the source line bias. This means a higher controlgate voltage need be applied to compensate for the bias. In FIG. 8, thesource line bias results in a shifting of the distribution (broken line)towards a higher supplied V_(T). The shifting will be more for that ofthe higher (lower current) memory states. If the breakpoint 381 isdesigned for the case without source line error, then the existence of asource line error will have some of the tail end of “1” states havingconduction currents to appear in a region of no conduction, which meanshigher than the breakpoint 381. This will result in some of the “1”states (more conducting) being mistakenly demarcated as “2” states (lessconducting.)

For example, the present multi-pass sensing can be implement in twopasses (j=1 to 2). After the first pass, those memory cells withconduction currents higher than the breakpoint 381 are identified andremoved by turning off their conduction current. A preferred way to turnoff their conduction currents is to set their drain voltages on the bitlines to ground. Referring also to FIG. 7A, this will effective removeall the higher current states demarcated by the breakpoint 381,resulting in a much reduced i_(TOT) and therefore a much reducedV_(drop). In a second pass (j=2), because of the removal of the highcurrent states that contributed to the source line bias, thedistribution with the broken line approaches that of the one with thesolid line. Thus sensing using the breakpoint 381 as the demarcationcurrent value will not result in mistaking the “1” states for the “2”states.

As compared to a conventional one-pass approach, the present two-passmethod substantially reduces the likelihood of misidentifying some ofthe “1” cells as “2” or higher cells. More than two passes are alsocontemplated, although there will be diminishing returns with increasingnumber of passes. Further each pass may have the same demarcationcurrent, or with each successive pass, the demarcation current usedconverges to that of a breakpoint normally used in a conventional singlepass sensing.

FIG. 9 is a flow diagram showing a multi-pass sensing method forreducing source line bias, according to one embodiment of the invention.

-   -   STEP 400: For a page of memory cells, initially set an operating        set of memory cells equal to the page of memory cells.    -   STEP 410: Begin the multi-pass j=1 to N.    -   STEP 420: Set a demarcation current value, I₀(j), where after        the first pass j>1, I₀(j) is less than or equal that of a        previous pass j−1, i.e. I₀(j)<=I₀(j−1).    -   STEP 430: Determine those memory cells among the operating set        having a conduction current higher than the demarcation current        value I₀(j).    -   STEP 440: Inhibit further current flow in those memory cells        with a conduction current higher than the demarcation current        value I₀(j).    -   STEP 450: Set the operating set of memory cells equal to the        remaining memory cells whose conduction currents have not been        inhibited. If j<N, return to STEP 410, otherwise proceed to STEP        460.    -   STEP 460: Read out the states of the page of memory cells.    -   STEP 470: End.

FIG. 10 is a schematic diagram illustrating a multi-pass sense module,according to a preferred embodiment of the invention. The multi-passsense module 380 senses the conduction current of a memory cell 10 via acoupled bit line 36. It has a sense node 481 that can be selectivelyconnected a number of components. Initially, an isolation transistor482, when enabled by a signal BLS connects the bit line 36 to the sensenode 381. A precharge circuit 484 is coupled to the sense node 481. Whenthe precharged circuit 484 is enabled, it brings the bit line voltage toa predetermined drain voltage appropriate for sensing. At the same time,the control gate of the memory cell is set to a predetermined thresholdvoltage V_(T)(i) for a given memory state under consideration. This willinduce a source-drain conduction current to flow in the memory cell 10,which may be sensed from the coupled bit line 36. The conduction currentis a function of the charge programmed into the memory cell and theapplied V_(T)(i) when there exists a norminal voltage difference betweenthe source and drain of the memory cell.

A sense amplifier 390 is then connected to the sense node to sense theconduction current in the memory cell 10. A cell current discriminator394 serves as a discriminator or comparator of current levels. Itdetermines whether the conduction current is higher or lower than agiven demarcation current value I₀(j). If it is higher, a latch 396 isset to a predetermined state. A pull-down circuit 486 is activiated inresponse to the latch 396 being set to the predetermined state, e.g.,with INV being HIGH. This will pull down the sense node 481 andtherefore the connected bit line 36 to ground voltage. This will inhibitthe conduction current flow in the memory cell 10 irrespective of thecontrol gate voltage since there will be no voltage difference betweenits source and drain.

In general, there will be a page of memory cells being operated on by acorresponding number of multi-pass sense modules 380. A page controller498 supplies control and timing signals to each of the sense modules. Inone embodiment, the page controller 498 is implemented as part of thestate machine 312 in the control circuitry 310 shown in FIG. 6A. Inanother embodiment, the page controller is part of the read/writecircuits 370. The page controller 498 cycles each of the multi-passsense module 380 through a predetermined number of passes (j=1 to N) andalso supplies a predetermined demarcation current value I₀(j) for eachpass. As will be seen in connection with FIG. 13 later, the demarcationcurrent value can also be implemented as a time period for sensing.After the last pass, the page controller 498 enables a transfer gate 488with a signal NCO to read the state of the sense node 481 as sensed datato a readout bus 499. In all, a page of sense data will be read out fromall the multi-pass modules 380.

FIG. 11 is a flow diagram showing the operation of the multi-pass sensemodule of FIG. 10.

-   -   STEP 400: For a page of memory cells each having a bit line        coupled to it, initially set an operating set of memory cells        equal to the page of memory cells.    -   STEP 402: Charge individual bit lines of the operating set of        memory cells to within a predetermined voltage range.    -   STEP 410: Begin the multi-pass j=1 to N.    -   STEP 412: Begin with individual bit lines of the operating set        of memory cells at a voltage within the predetermined voltage        range.    -   STEP 420: Set a demarcation current value, I₀(j), where after        the first pass j>1, I₀(j) is less than or equal that of a        previous pass j−1, i.e. I₀(j)<=I₀(j−1).    -   STEP 430: Determine those memory cells among the operating set        having a conduction current higher than the demarcation current        value I₀(j).    -   STEP 440: Inhibit further current flow in those memory cells        with a conduction current higher than the demarcation current        value I₀(j).    -   STEP 452: Set the operating set of memory cells equal to the        remaining memory cells whose bit lines have not been latched and        pulled to ground. If j<N, return to STEP 410, otherwise proceed        to STEP 460.    -   STEP 460: Read out the states of the page of memory cells.    -   STEP 470: End.        Sensing with Control of Bit Line to Bit Line Coupling

FIG. 12 illustrates three adjacent bit line and the effect of capacitivecouplings between them. A memory cell 10-0 has two adjacent memorycells, 10-1 and 10-2. Similarly, coupled to the three memory cells arerespectively three adjacent bit lines 36-0, 36-1 and 36-2. Each of thebit lines has its own self capacitance C_(BL0), C_(BL1) and C_(BL2)respectively. The pair of adjacent bit lines 36-0 and 36-1 has mutualcapacitance C_(BL01). The pair of adjacent bit lines 36-0 and 36-1 hasmutual capacitance C_(BL02).

It can then be seen, there could be various branches of current flow dueto the various capacitances. In particular, the currents due to each bitline self capacitance will result in:i _(BLC0) =C _(BL0) d/dt V _(BL0),i _(BLC1) =C _(BL1) d/dt V _(BL1)i _(BLC2) =C _(BL2) d/dt V _(BL2)

Similarly, the cross current due to the pair of adjacent bit lines 36-0and 36-1 is:i _(BLC01) =C _(BL01) d/dt(V _(BL0) −V _(BL1)), andi _(BLC02) =C _(BL02) d/dt(V _(BL0) −V _(BL2)).

For the memory cell 10-0, the cell's conduction current is:i _(CELL) ˜i _(BL0) +[i _(BLC00) +i _(BLC01) +i _(BLC02)].

The cell current given above is an approximation since it only includescontributions from adjacent bit lines. In general, for the bit line BL0there will also be capacitance C_(BL03) due to the non adjacent bitlines to the left as well as capacitance C_(BL04) due to the nonadjacent bit lines to the right. Similarly, there will be a mutualcapacitance C_(BL12) between non adjacent bit lines BL1 and BL2. Thesecapacitance will contribute to displacement current flow dependent on avarying voltage across each capacitor. It has been estimated that thecontributions from non adjacent bit lines amount to about ten percent ofthe contribution from the adjacent bit lines.

Also, since the sense module 380 is coupled to the bit line (see FIG.10), the current it detected is i_(BL0), which is not identical toi_(CELL), because of the current contributions from the various bit linecapacitances.

One prior art solution is to sense a memory cell while grounding the bitlines of adjacent cells. The conduction current in the memory cell issensed by noting the rate of discharge through the capacitance of thecoupled bit line. Thus, the conduction current can be derived from therate of change of the bit line voltage. Referring to FIG. 12, this meansthat while the conduction current on the bit line BL0 36-0 is beingsensed, the voltage V_(BL1) on adjacent bit line BL1 36-1 and V_(BL2) onadjacent bit line BL2 36-2 are set to zero. By shutting down thecurrents in adjacent bit lines, the crosstalk between adjacent bit linesis eliminated. However, since this prior art sensing results in a timevarying V_(BL0)=V_(BL0)(t), and by the equations given above, theself-capacitance of BL0 with respect to ground becomesC_(BL00)+C_(BL01)+C_(BL02). This prior art sensing also does noteliminate displacement currents contributed from the non adjacent bitlines such as those associated with C_(BL03), C_(BL04), and C_(BL12).These currents are smaller in magnitude, but nevertheless appreciable.

According to another aspect of the present invention, a memory deviceand a method thereof provide sensing a plurality of memory cells inparallel while minimizing errors caused by bit-line to bit-linecoupling. Essentially, the bit line voltages of the plurality of bitline coupled to the plurality of memory cells are controlled such thatthe voltage difference between each adjacent pair of line lines issubstantially independent of time while their conduction currents arebeing sensed. When this condition is imposed, all the currents due tothe various bit line capacitance drop out since they all depend on atime varying voltage difference. Thus, from the equation above, since[i_(BLC00)+i_(BLC01)+i_(BLC02)]=0, the current sensed from the bit lineis identical to the cell's current, e.g., i_(BL0)=i_(CELL).

FIG. 13A is a flow diagram showing a method of sensing while reducingbit-line to bit-line coupling.

-   -   STEP 500: Couple a bit line to each of a page of memory cells in        order to sense their conduction currents.    -   STEP 510: Charge each bit line to a bit line voltage within a        predetermined voltage range.    -   STEP 520: Control the bit line voltage of each bit line such        that the voltage difference between each adjacent pair of bit        lines is substantially independent of time.    -   STEP 530: While the bit lines are under control, sense the        conduction current through each bit line.    -   STEP 540: End.

According to another aspect of the present invention, in spite of theconstant voltage condition, a sensing circuit and method allowdetermination of the memory cell's conduction current by noting the rateof voltage change of a given capacitor.

FIG. 13B is a flow diagram showing a more detailed embodiment of thesensing step 530 shown in FIG. 13A.

-   -   STEP 532: While the bit lines are under control, sense the        conduction current through each bit line by using it to change        the voltage across a given capacitor.    -   STEP 534: Determine the conduction current by the rate of change        in the voltage across the given capacitor.

FIG. 14 illustrates a preferred sense module implementing the variousaspects of the present invention. The sense module 480 comprises a bitline isolation transistor 482, a bit line pull down circuit 486, a bitline voltage clamp 610, a readout bus transfer gate 488 and a senseamplifier 600.

The sense module 480 is connectable to the bit line 36 of a memory cell10 when the bit line isolation transistor 482 is enabled by a signalBLS. The sense module 480 senses the conduction current of the memorycell 10 by means of the sense amplifier 600 and latches the read resultas a digital voltage level SEN2 at a sense node 481 and outputs it to areadout bus 499.

The sense amplifier 600 essentially comprises a second voltage clamp620, a precharge circuit 640, a discriminator or compare circuit 650 anda latch 660. The discriminator circuit 650 includes a dedicatedcapacitor 652.

The sense module 480 is similar to the multi-pass sense module 380 shownin FIG. 10. However, in FIG. 14, the precharge circuit 640 isimplemented with a weak pull-up feature as will be described later. Thisserves as another way of identifying those cells with higher currents inorder to turn them off for the purpose of reducing source line biaserror.

The sense module 480 also has additional features for reducing bit-lineto bit-line coupling. This is implemented by keeping the bit linevoltage time-independent during sensing. This is accomplished by the bitline voltage clamp 610. As will be described below, the second voltageclamp 620 ensures the proper functioning of the bit line voltage clamp610 under all sensing conditions. Also sensing is not done by the priorart method of noting the rate of discharging the capacitance of the bitline due to the conduction current, but the rate of discharging thededicated capacitor 652 provided by the sense amplifier 600.

One feature of the sense module 480 is the incorporation of a constantvoltage supply to the bit line during sensing in order to avoid bit lineto bit line coupling. This is preferably implemented by the bit linevoltage clamp 610. The bit line voltage clamp 610 operates like a diodeclamp with a transistor 612 in series with the bit line 36. Its gate isbiased to a constant voltage BLC equal to the desired bit line voltageV_(BL) above its threshold voltage V_(T). In this way, it isolates thebit line from the sense node 481 and set a constant voltage level forthe bit line, such as the desired V_(BL)=0.5 to 0.7 volts. In generalthe bit line voltage level is set to a level such that it issufficiently low to avoid a long precharge time, yet sufficiently highto avoid ground noise and other factors.

The sense amplifier 600 senses the conduction current through the sensenode 481 and determines whether the conduction current is above or belowa predetermined value. The sense amplifier outputs the sensed result ina digital form as the signal SEN2 at the sense node 481 to the readoutbus 499.

The digital control signal INV, which is essentially an inverted stateof the signal SEN2, is also output to control the pull down circuit 486.When the sensed conduction current is higher than the predeterminedvalue, INV will be HIGH and SEN2 will be LOW. This result is reinforcedby the pull down circuit 486. The pull down circuit 486 includes ann-transistor 487 controlled by the control signal INV.

The operation and timing of the sense module 480 will be described byreference to both FIG. 14 and the timing diagrams FIGS. 15(A)-15(K).FIGS. 15(A)-15(K) are demarcated into PHASES (1)-(9).

PHASE (0): The sense module 480 is connected to the bit line 36 via anenabling signal BLS (FIG. 15(A)(0).) The Voltage clamp is enabled withBLC. (FIG. 15(B)(0).) The Precharge circuit 640 is enabled as a limitedcurrent source with a control signal FLT (FIG. 15(C)(0).)PHASE (1): Controlled Precharge

The sense amplifier 600 is initialized by a reset signal RST (FIG.15(D)(1)) which will pull the signal INV to ground via the transistor658 Thus on reset, INV is set to LOW. At the same time, a p-transistor663 pulls a complimentary signal LAT to V_(dd) or HIGH (FIG. 15(F)(1).)

The isolation gate 630 is formed by an n-transistor 634, which iscontrolled by the signal LAT. Thus after reset, the isolation gate isenabled to connect the sense node 481 to the sense amplifier's internalsense node 631, and the signal SEN2 will be the same as the signal SENat the internal sense node 631.

The precharge circuit 640 precharges the bit line 36 through theinternal sense node 631 and the sense node 481 for a predeterminedperiod of time. This will bring the bit line to an optimum voltage forsensing the conduction therein.

The precharge circuit 640 includes a pull-up p-transistor 642 controlledby the control signal FLT (“FLOAT”.) The bit line 36 will be pulled uptowards the desired bit line voltage as set by the bit line voltageclamp 610. The rate of pull-up will depend on the conduction current inthe bit line 36. The smaller the conduction current, the faster thepull-up.

FIGS. 15(H1)-15(H4) illustrate respectively the bit line voltages ofmemory cells having conduction currents of 700 nA, 400 nA, 220 nA and 40nA.

It has been described earlier in connection with FIGS. 7-11 that sensingerrors due to the source line bias are minimized if those memory cellswith conduction currents higher than a predetermined value are turnedoff and their contributions to the source line bias eliminated.

According to another aspect of the invention, the precharge circuit 640is implemented to serve two functions. One is to precharge the bit lineto an optimum sensing voltage. The other is to help identify thosememory cells with conduction currents higher than a predetermined valuefor D.C. (Direct Current) sensing so that they may be eliminated fromcontributing to source line bias.

The D.C. sensing is accomplished by providing a precharge circuit thatbehaves like a current source for supplying a predetermined current tothe bit line. The signal FLT that controls the p-transistor 642 is suchthat it “programs” a predetermined current to flow through the prechargecircuit 640. As an example, the FLT signal may be generated from acurrent mirror with a reference current set to 500 nA. When thep-transistor 642 forms the mirrored leg of the current mirror, it willalso have the same 500 nA throwing in it.

FIGS. 15(I1)-15(I4) illustrate the voltages on four example bit linesconnected respectively to memory cells with conduction currents of 700nA, 400 nA, 220 nA and 40 nA. When the precharge circuit 640 is acurrent source with a limit of 500 nA, for example, a memory cell havinga conduction current exceeding 500 nA will have the charges on the bitline drained faster than it can accumulate. Consequently, for the bitline with conduction current 700 nA, its voltage or the signal SEN atthe internal sense node 631 will remain close to 0 v (FIG. 15(I1)(1).)On the other hand, if the memory cell's conduction current is below 500nA, the precharge circuit 640 will begin to charge up the bit line andits voltage will begin to rise towards the clamped bit line voltage(e.g., 0.5 v set by the voltage clamp 610). (FIGS. 15(I2)(1)-15(I4)(1).)Correspondingly, the internal sense node 631 will either remain close to0 v or be pulled up to V_(dd) (FIG. 15(G).) Generally, the smaller theconduction current, the faster the bit line voltage will charge up tothe clamped bit line voltage. Thus, by examining the voltage on a bitline after the controlled precharge phase, it is possible to identify ifthe connected memory cell has a conduction current higher or lower thana predetermined level.

PHASE (2): D.C. Latching & Removing the High Current Cells fromSubsequent Sensing

After the controlled precharge phase, an initial, D.C. high-currentsensing phase begins where the signal SEN is sensed by the discriminatorcircuit 650. The sensing identifies those memory cells with conductioncurrents higher than the predetermined level. The discriminator circuit650 includes two p-transistors 654 and 656 in series, which serve as apull up for a node 657 registering the signal INV. The p-transistor 654is enabled by a read strobe signal STB going LOW and the p-transistor656 is enabled by the SEN signal at the internal sense node 631 goingLOW. As explained earlier, the high current cells will have the signalSEN close to 0 v or at least unable for its bit lines to be prechargedsufficient high to turn off the p-transistor 656. For example, if theweak pull up is limited to a current of 500 nA, it will fail to pull upa cell with conduction current of 700 nA (FIG. 15 (G1)(2).) When STBstrobes LOW to latch, INV at the node 657 is pulled up to V_(dd). Thiswill set the latch circuit 660 with INV HIGH and LAT LOW (FIG. 15(H1)(2).)

When INV is HIGH and LAT LOW, the isolation gate 630 is disabled and thesense node 481 is blocked from the internal sense node 631. At the sametime, the bit line 36 is pulled to ground by the pull down circuit 486(FIG. 15(I1)(2).) This will effective turn off any conduction current inthe bit line, eliminating it from contributing to source line bias.

Thus, in one preferred implementation of the sense module 480, alimited-current source precharge circuit is employed. This provides anadditional or alternative way (D.C. sensing) to identify bit linescarrying high currents and to turn them off to minimize source line biaserror in subsequent sensing.

In another embodiment, the precharge circuit is not specificallyconfigured to help identify high current bit lines but is optimized topull up and precharge the bit line as fast as possible within theallowance of the maximum current available to the memory system.

PHASE (3): Recovery/Precharge

Prior to a sensing of the conduction current in a bit line such as bitline 36 that has not been previously pulled down, the precharge circuitis activated by the signal FLT to precharge the internal sense node 631to V_(dd) (FIG. 15(C)(3) and FIGS. 15(I2)(3)-15(I4)(3).)

PHASE (4): 1^(st) A.C. Sensing

The operation from here onwards is similar to the multi-pass sensingdescribed in connection with FIGS. 10-11 in that the sense node isfloated and its voltage changes during current sensing (A.C. orAlternate Current sensing.) The enhancement in FIG. 14 is that thesensing is performed with the bit line voltage kept constant to avoidbit-line to bit-line coupling.

In a preferred embodiment, an A.C. (Alternate Current) sensing isperformed by determining the voltage drop at the floated internal sensenode 631. This is accomplished by the discriminator or compare circuit650 employing the capacitor C_(SA) 652 coupled to the internal sensenode 631, and considering the rate the conduction current is dischargingit. In an integrated circuit environment, the capacitor 652 is typicallyimplemented with a transistor. It has a predetermined capacitance, e.g.,30 fF, which can be selected for optimum current determination. Thedemarcation current value, typically in the range 100-1000 nA, can beset by appropriate adjustment of the discharging period.

The discriminator circuit 650 senses the signal SEN in the internalsense node 631. Prior to each sensing, the signal SEN at the internalsense node 631 is pull up to V_(dd) by the precharge circuit 640. Thiswill initially set the voltage across the capacitor 652 to be zero.

When the sense amplifier 600 is ready to sense, the precharge circuit640 is disabled by FLT going HIGH (FIG. 15(C)(4).) The first sensingperiod T1 is set by the assertion of the strobe signal STB. During thesensing period, a conduction current induced by a conducting memory cellwill discharge the capacitor. SEN will decrease from V_(dd) as thecapacitor 652 is discharging through the draining action of theconduction current in the bit line 36. FIGS. 15(G2)-15(G4) illustraterespectively the SEN signal corresponding to the remaining three examplebit lines connected respectively to memory cells with conductioncurrents of 400 nA, 220 nA and 40 nA. The decrease being more rapid forthose with a higher conduction current.

PHASE (5): 1^(st) A.C. Latching and Removal of Higher Current Cells fromSubsequent Sensing

At the end of the first predetermined sensing period, SEN will havedecreased to some voltage depending on the conduction current in the bitline 36 (FIGS. 15(G2)(4)-15(G4)(4).) As an example, the demarcationcurrent in this first phase is set to be at 300 nA. The capacitor C_(SA)652, the sensing period T1 and the threshold voltage of the p-transistor656 are such that the signal SEN corresponding to a conduction currenthigher that the demarcation current (e.g., 300 nA) will drop sufficientlow to turn on the transistor 656 in the discriminator circuit 650. Whenlatching signal STB strobes LOW, the output signal INV will be pulledHIGH, and will be latched by the latch 660 (FIG. 15(E)(5) and FIG.15(H2).) On the other hand, the signal SEN corresponding to a conductioncurrent below the demarcation current will produce a signal SEN unableto turn on the transistor 656. In this case, the latch 660 will remainunchanged, in which case LAT remains HIGH (FIGS. 15(H3) and 15(H4).)Thus it can be seen that the discriminator circuit 650 effectivelydetermines the magnitude of the conduction current in the bit line 36relative to a reference current set by the sensing period.

The sense amplifier 600 also includes the second voltage clamp 620 whosepurpose is to maintain the voltage of the drain of the transistor 612sufficiently high in order for the bit line voltage clamp 610 tofunction properly. As described earlier, the bit line voltage clamp 610clamps the bit line voltage to a predetermined value V_(BL), e.g., 0.5v. This will require the gate voltage BLC of the transistor 612 to beset at V_(BL)+V_(T) (where V_(T) is the threshold voltage of thetransistor 612) and the drain connected to the sense node 481 to begreater than the source, i.e., the signal SEN2>V_(BL). In particular,given the configurations of the voltage clamps 610 and 620, SEN2 shouldbe no higher than the smaller of (LAT-V_(T)) or (BLX-V_(T)), and SENshould be no lower. During sensing, the isolation gate 630 is in apass-through mode. However, during sensing the signal SEN at theinternal sense node 631 has a voltage that decreases from V_(dd). Thesecond voltage clamp 620 prevents SEN from dropping to (LAT-V_(T)) or(BLX-V_(T)), whichever is lower. This is accomplished by an n-transistor612 controlled by a signal BLX, where BLX is ≧V_(BL)+2V_(T) (FIG.15(F).) Thus, through the actions of the voltage clamps 610 and 620, thebit line voltage V_(BL) is kept constant, e.g.˜0.5 v during sensing.

Measuring current using a dedicated capacitor 652 instead of prior art'suse of the bit line capacitance is advantageous in several respects.First, it allows a constant voltage source on the bit line therebyavoiding bit-line to bit-line crosstalk. Secondly, the dedicatedcapacitor 652 allows a capacitance to be selected that is optimal forsensing. For example, it may have a capacitance of about 30 fF ascompared to a bit line capacitance of about 2 pF. A smaller capacitancecan increase the sensing speed since it discharges faster. Finally,sensing relative to a dedicated capacitance as compared to the prior artmethod of using the capacitance of the bit line allows the sensingcircuits to be independent of the memory architecture.

In another embodiment, the current determination is accomplished bycomparison with a reference current, which may be provided by theconduction current of a reference memory cell. This could be implementedwith the compare current as part of a current mirror.

The output of the current determination LAT is latched by the latchcircuit 660. The latch circuit is formed as a Set/Reset latch by thetransistors 661, 662, 663, and 664 together with the transistors 666 and668. The p-transistor 666 is controlled by the signal RST (RESET) andthe n-transistor 668 is controlled by the signal STB (STROBE or SET*.)

In general, there will be a page of memory cells being operated on by acorresponding number of multi-pass sense modules 480. For those memorycells having conduction current higher than the first demarcationcurrent level, their LAT signal will be latch LOW. This in turnsactivates the bit line pull down circuit 486 to pull the correspondingbit lines to ground, thereby turn off their currents.

PHASE (6): Recovery/Precharge

Prior to the next sensing of the conduction current in a bit line suchas bit line 36 that has not been previously pulled down, the prechargecircuit is activated by the signal FLT to precharge the internal sensenode 631 to V_(dd) (FIG. 15(C)(6) and FIGS. 15(I3)(6)-15(I4)(6).)

PHASE (7): 2^(nd) Sensing

When the sense amplifier 600 is ready to sense, the precharge circuit642 is disabled by FLT going HIGH (FIG. 15(C)(7).) The second sensingperiod T2 is set by the assertion of the strobe signal STB. During thesensing period, a conduction current, if any will discharge thecapacitor. SEN will decrease from V_(dd) as the capacitor 652 isdischarging through the draining action of the conduction current in thebit line 36.

In accordance with the example before, the memory cells with conductioncurrents higher than 300 nA have already been identified and shut downin the earlier phases. FIGS. 15(G3)(7) and 15(G4)(7) illustraterespectively the SEN signal corresponding to the two example bit linesconnected respectively to memory cells with conduction currents of 220nA and 40 nA.

PHASE (8): 2^(nd) Latching for Reading Out

At the end of the second predetermined sensing period T2, SEN will havedecreased to some voltage depending on the conduction current in the bitline 36 (FIGS. 15(G3)(7)-15(G4)(7).) As an example, the demarcationcurrent in this second phase is set to be at 100 NA. In this case, thememory cell with the conduction current 220 nA will have its LAT latchedLOW (FIG. 15(H3)(7)) and its bit line subsequently pulled to ground(FIG. 15(I3)(7).) On the other hand, the memory cell with the conductioncurrent 40 nA will have no effect on the state of the latch, which waspreset with LAT HIGH.

PHASE (9): Read Out to the Bus

Finally, in the read out phase, the control signal NCO at the transfergate 488 allows the latched signal SEN2 to be read out to the readoutbus 499 (FIGS. 15(J) and 15(K).)

A page controller such as the page controller 398 also shown in FIG. 10supplies control and timing signals to each of the sense modules.

As can be seen from FIGS. 15(I1)-15(I4), the bit line voltage remainsconstant during each sensing period. Thus, from the discussion early,capacitive bit-line to bit-line coupling is eliminated.

The sense mode 480 shown in FIG. 14 is one preferred embodiment wheresensing is performed with three passes. The first two passes beingimplemented to identify and shut down higher current memory cells. Withthe higher current contributions to the source line bias eliminated, thefinal pass is able to sense the cells with lower range conductioncurrents more accurately.

In other embodiments, sensing operations are implemented with differentcombination of D.C. and A.C. passes. Some even using only two or moreA.C. passes. For the different passes, the demarcation current valueused may be the same each time or converge progressively towards thedemarcation current used in the final pass.

Management of the Errors Introduced by Neighboring Floating GateCoupling

Another error inherent in high density integrated circuit, non-volatilememory device is due to neighboring floating gate coupling, as describedearlier. The close proximity of the memory cells causes fieldperturbations from the charge elements of neighboring cells. Accordingto another aspect of the present invention, the errors due to theperturbations are minimized by minimizing the change in the fieldenvironment of each cell between programming and reading. This isaccomplished by programming all adjacent memory cells in a page thereoftogether. Since the individual memory cells and their neighbors areprogrammed together, it will ensure a minimum change in fieldenvironment seen by the individual cells from the time they areprogrammed to the time they are read.

This is in contrast to the prior art case of programming even and oddpages independently. In that case, after the memory cells of an evenpage have been programmed, the field contributed by their adjacentmemory cells in an odd page may have changed radically when the odd pageis programmed with a different set of data.

As described earlier, the number of memory cells in a “page” that areprogrammed or read simultaneously may vary according to the size of datasent or requested by a host system. Thus, there are several ways toprogram the memory cells coupled to a single word line, such as (1)programming even bit lines and odd bit lines separately, which maycomprise upper page programming and lower page programming, (2)programming all the bit lines (“all-bit-line programming”), or (3)programming all the bit lines in a left or right page separately, whichmay comprise right page programming and a left page.

In existing non-volatile memory devices, a row of memory cells joined bythe same word lines is configured into two interleaving pages. One pageconsists of memory cells of the even columns and the other page consistsof memory cells of the odd columns. The even or odd pages are separatelysensed and programmed. As mentioned earlier, this is necessitated by theneed to control bit-line to bit-line coupling. Thus, it is preferable toground alternate bit lines while read/write operations are performed onthe other set of the bit lines.

However, as mentioned earlier, the interleaving page architecture isdisadvantageous in at least three respects. First, it requiresadditional multiplexing circuitry. Secondly, it is slow in performance.To finish read or program of memory cells connected by a word line or ina row, two read or two program operations are required. Thirdly, it isalso not optimum in reducing other disturb effects such as fieldcoupling from neighboring charge storage elements.

All Bit Line Programming

As described in connection with FIGS. 12-15, it is possible by thepresent invention to control bit-line to bit-line coupling. Thus, thereis no need to ground alternate bit lines during sensing or programverify, thereby relaxing the requirement to operate on even or odd pageswith non-contiguous memory cells and speeding up verify operations.

According to another aspect of the invention, a contiguous page ofmemory cells are programming in parallel while bit-line to bit-linecoupling is under control. This will minimize the extraneous fieldeffects from neighboring floating gates.

The sense module shown in FIG. 6A, FIG. 10 and FIG. 14 is preferablyimplemented in a memory architecture configured to perform all-bit-linesensing. In other words, contiguous memory cells in a row are eachconnectable to a sense module to perform sensing in parallel. Such amemory architecture is also disclosed in co-pending and commonlyassigned United States patent application, “Highly Compact Non-VolatileMemory And Method Thereof,” by Raul-Adrian Cernea, filed on the same dayas the present application. The entire disclosure of said patentapplication is hereby incorporated herein by reference.

FIG. 16A is a flow diagram showing a method of programming and readingthat reduces the errors due to neighboring floating gate coupling.

-   -   STEP 700: Program and verify a page of memory cells in parallel        in such a way that the difference in effective electric field        experienced by individual memory cells during a last program        verify and a subsequent read is minimized.    -   STEP 710: End.

FIG. 16B is a flow diagram showing a preferred embodiment of theinventive step shown in FIG. 16A.

-   -   STEP 730: Form a page of contiguous memory cells.    -   STEP 740: Program and verify the page of memory cells in        parallel.    -   STEP 750: Subsequently, read the page of memory cells    -   STEP 760: End.        Programming Left and Right Pages

FIG. 17 illustrates a memory array similar to that shown in FIGS. 6A and6B, except its architecture has each row of memory cells organized intoa left pages 301 and a right page 302 of memory cells. Each pageconsists of a plurality of contiguous memory cells. For example, eachpage may have 4,256 cells. In the preferred embodiment, programming isperformed on the left page and the right page individually. To minimizeinteraction between the two independent pages, while one page is beingprogrammed, the other page has all its bit lines grounded. Again, byhaving each page contiguous, neighboring floating gate coupling isreduced during programming.

Although the various aspects of the present invention have beendescribed with respect to certain embodiments, it is understood that theinvention is entitled to protection within the full scope of theappended claims.

1. In a non-volatile memory device having a plurality of memory cells tobe sensed in parallel, each memory cell having a source electrode, andthe plurality of memory cells having their source electrodes tiedtogether into a combined source line, and each memory cell having adrain electrode coupled to an associated bit line, a method of sensingcomprising: (a) providing a predetermined demarcation current value todiscriminate between two memory states; (b) providing a pulldown circuitfor each associated bit line; (c) sensing the plurality of memory cellsin parallel; (d) identifying those memory cells having conductioncurrents higher than said predetermined demarcation current value; (e)turning off the conduction currents of those higher current memory cellsby enabling the associated pulldown circuits to ground the associatedbit lines; (f) repeating (c) to (e) for a predetermined number of times;and (g) sensing the plurality of memory cells in parallel in a finalpass.
 2. The method of claim 1, wherein said predetermined number oftimes is zero.
 3. The method of claim 1, wherein said predeterminednumber of times is one or greater.
 4. The method of claim 1, wherein:the conduction currents of said plurality of memory cells are sensedthough a plurality of associated bit lines; and said step of identifyingthose high current memory cells includes: precharging said plurality ofassociated bit lines with a constant current source having apredetermined current limitation; identifying those memory cells havingconduction currents higher than said predetermined demarcation currentvalue by their associated bit lines' precharging rates.
 5. The method ofclaim 1, wherein: said sensing is performed by an associated pluralityof sense amplifier in parallel; and said step of identifying those highcurrent memory cells includes: identifying those memory cells by usingtheir associated sense amplifiers to compare their conduction currentsrelative to said predetermined demarcation current value.
 6. The methodof claim 1, wherein: said plurality of memory cells is non-volatilememory.
 7. The method of claim 1, wherein: said plurality of memorycells is flash BEPROM.
 8. In a non-volatile memory device having aplurality of memory cells to be sensed in parallel, each memory cellhaving a source electrode, and the plurality of memory cells havingtheir source electrodes tied together into a combined source line, andeach memory cell having a drain electrode coupled to a bit line, a readsystem comprising: a controller; and a plurality of sensing circuits forsensing said plurality of memory cells in parallel, each sensing circuitfurther comprising: a discriminator coupled to receive a conductioncurrent of an associated memory cell, said discriminator discriminatingwhether the conduction current is higher or lower than a predetermineddemarcation current value; a latch being set to register the associatedmemory cell in response to said discriminator identifying a conductioncurrent higher than said predetermined demarcation current value; and apulidown circuit being enabled by said latch being set to pull the bitline of the associated memory cell to ground; and wherein saidcontroller controls said plurality of sensing circuits to operate apredetermined number of times before reading out the determined memorystate.
 9. The read circuit as in claim 8, wherein said predeterminednumber of times is two.
 10. The read circuit as in claim 8, wherein saidpredetermined number of times is greater than two
 11. The read circuitas in claim 8, further comprising: a precharge circuit which is aconstant current source having a predetermined current limitation; andwherein: the conduction currents of said plurality of memory cells aresensed though a plurality of associated bit lines; said prechargecircuit pre charging said plurality of associated bit lines; and saidplurality of sensing circuits identifying those memory cells havingconduction currents higher than said predetermined demarcation currentvalue by their associated bit lines' precharging rates.
 12. The readcircuit as in claim 8, wherein: said plurality of sensing circuitsidentifying those memory cells having conduction currents by comparingtheir conduction currents relative to said predetermined demarcationcurrent value.
 13. The read circuit as in claim 8, wherein: saidplurality of memory cells is non-volatile memory.
 14. The read circuitas in claim 8, wherein: said plurality of memory cells is flash EEPROM.